Cooking oils, such as those used in commercial deep fryers, tend to become contaminated with such things as moisture, food cell bodies, and food plasmas during frying. The oils also tend to break down chemically after extended use, often causing the oil to foam, smoke, smell bad, look bad, or taste bad. It has been found that keeping the cooking oil clean by filtering it on a regular basis tends to extend the useful life of the cooking oil, and increases the quality and appearance of foods which are cooked therein.
A variety of specialized systems and filter media for filtering cooking oil have been developed. Ideally, such filtering systems would remove contaminants from the cooking oil down to a level of about one micron. However, in large-scale industrial applications, such as commercial deep fryers used to cook potato chips or precook French fries, this desired level of ultrafiltration has not been feasible. Current filtering systems being used in this area of application may be broadly segregated into two types--absolute filtering systems and depth filtration systems.
Absolute filtering systems employ a filter medium such as paper or a metal screen (wire cloth), the medium having a discrete pore size. Contaminated cooking oil may be moved through the filter medium by gravity, positive pressure, or vacuum. However, gravity feed is by far the most common due to economic considerations.
While ultrafiltration levels approaching one micron are possible with absolute filtering systems, multiple stages of filter media having incrementally smaller pore sizes must be employed. Such an approach yields an arduous and costly filtering system. Furthermore, such absolute filtering systems possess a poor loading capacity, thereby requiring frequent changeouts of the filter medium. Of course, such changeouts have the detrimental effect of a complete stop in production. Additionally, if a metal screen medium is utilized, cleaning such a screen can be a difficult and time consuming process. The result of these limitations is that, if ultrafiltration approaching one micron is desired, large volumes of contaminated oil simply cannot be economically handled.
Depth filtration systems utilize a filter medium having a substantial depth dimension, which thereby allows contaminants to be removed throughout the entire depth of the medium. This yields an increased loading capacity relative to absolute filtration. Contaminated cooking oil may be moved through the filter medium by positive pressure, as is found, for instance, in plate-and-frame filtering systems. Contaminated cooking oil may also be moved through the filter medium under a vacuum, as is found in rotary vacuum filtering systems. Both systems typically employ the use of a filtering powder (e.g., diatomaceous earth) that is added to the contaminated oil. The filtering powder, which is suspended in the contaminated oil, begins to plate out over a septum (e.g., the porous drum in the rotary vacuum system) during the initial stages of filtering. When a sufficient layer of filtering powder has so formed, a depth filter medium is created over the top of the septum. Gravity feed is not feasible with depth filtration systems, as a force greater than that of gravity is required to move the contaminated cooking oil through the filter medium.
Depth filtration systems utilized in large-scale industrial applications require very large and sophisticated filtering equipment, as well as a good deal of support equipment. Such systems are therefore very expensive. Additionally, while such depth filtration systems can handle large volumes of contaminated oil, they cannot economically achieve filtration levels below approximately 10 microns. These systems also require periodic removal of the filter medium, which leads directly to process down time. In fact, due to the complex nature of the equipment involved, down time may be even longer with such systems than is the case with absolute filtering systems. Additionally, the cleanup process may be complex and time consuming for depth filtration systems and, in the case of positive pressure systems, may be hazardous.
In many commercial fryers, contaminated cooking oil is never directly removed and replaced. Instead, the fryer is replenished with fresh cooking oil to account for that amount of cooking oil leaving the fryer with the product cooked therein. Viewed macroscopically, it can be said that all oil leaves with the product. "Turnover" is the time it takes to add replenishing oil in an amount equaling the fryer volume. Stated differently, turnover is simply the time it takes for a complete volume changeover to occur in the fryer.
It is well known that cooking oil quality drops off exponentially after a certain period of use. One representative reference describing a cooking oil's degradation profile is a paper entitled "Frying Theory and Practice" presented by Michael Blumenthal at the University of California-Davis on May 17, 1990. Given this degradation profile, fryers are designed such that turnover occurs prior to the time at which the oil quality begins to precipitously drop.
When an in-line filtering system is added to a fryer, the total system volume is increased. However, the amount of oil leaving the fryer with the product remains constant. The net result is that turnover for the total system is increased, which may be problematic if operation of the fryer is then conducted using heavily degraded oil. Thus, in many areas of application it is extremely advantageous to minimize the volume of the added in-line filtering system.
As a result, there has been a long-felt need for a filtering system and medium combination useful in filtering contaminated cooking oil that: (1) can handle the large volumes of contaminated cooking oil required in large-scale industrial applications; (2) achieves ultrafiltration levels near or below one micron; (3) exploits those attributes incident to depth filtration; (4) is relatively inexpensive to produce; (5) is self-contained, thereby requiring virtually no support equipment; (6) causes no excessive damage to the oil during filtering; (7) ensures proper orientation of the filter medium during the filtering process; and (8) minimizes the increase in total system volume, thereby minimizing the increase in turnover. A similar need exists in the large-scale commercial filtering of other contaminated fluids (e.g., motor oil, etc.). The present invention, when used in conjunction with a currently available filter medium, is directed to satisfying the above-described need.